When batteries are connected in parallel, as in most R.V’s, the positive
terminals are connected together and the negative terminals are connected together
as shown below:

The main advantage of these parallel connections is that the total current
delivering capacity is the sum of each battery’s current. The total current
is equal to the sum of both batteries. The main disadvantage of this type of
connection is that the total voltage available cannot exceed the voltage of
the weakest battery.

Unfortunately, too often we see parallel connections that may be less efficient.
For example, the diagram below shows the traditional way to connect the cables
to a dual-battery pack.

The positive and negative cables are connected to the terminals on the same
battery, battery #1. This puts more of the load on battery #1 for the cranking
mode and it also gets a higher charging voltage when the dual-battery pack is
being charged. Battery #2 doesn’t work as hard because it has extra resistance
to overcome in the cables that connect it to Batt #1. Battery #1 doesn’t
see that resistance so it is not affected. The cable and connection resistances
reduce the efficiency of battery #2 when cranking or charging especially if
corrosion develops. The figures shown below demonstrate more efficient ways
to connect the battery cables to a dual and triple-battery pack.